Command docs: diverse small fixes

Command docs: diverse small fixes

Better synopsis for 'openssl dgst' and 'openssl enc', correct names
for 'openssl rehash' ('c_rehash' is mentioned there too), correct
option end marker for 'openssl verify', and finally, refer to
sub-commands as sub-commands.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10065)